1. Field of the Invention
The present invention relates to a structure preferable as a backlight or a frontlight for use in a liquid crystal display device, and a liquid crystal display device provided with the backlight or the frontlight.
2. Description of the Related Art
FIG. 20 is a diagrammatic sectional view showing one example of an existing transmissive liquid crystal display device, and a liquid crystal display device 100 in the example comprises a liquid crystal display unit 120 and a backlight 110 disposed at a rear surface side of the liquid crystal display unit 120.
The backlight 110 is constituted so that light from a long light source 113 may be allowed to enter into a light guide plate 112 from one end surface (incidence surface 112a) thereof and to exit from one side surface (exit surface 112b) that faces the liquid crystal unit 120 in the light guide plate 112. On the other side surface 112c on a side that faces the exit surface 112b of the light guide plate 112, a prism sheet 114 and a white sheet 115 are disposed in this order. Furthermore, in order to integrally hold constituent components of the backlight 110, a cover-like holding member 116 that can accommodate in one lump the long light source 113, the light guide plate 112, the prism sheet 114 and the white sheet 115 is disposed. Furthermore, the white sheet 115 that is disposed on a rear surface side of the light guide plate 112 has light diffusion properties and light reflection properties, still furthermore between the light guide plate 112 and the white sheet 115 the prism sheet 114 is disposed, and thereby light diffusion properties and uniformity of a brightness distribution are improved. Furthermore, as the long light source 113, a cold cathode tube has been used.
However, in a backlight of a smaller display device such as an information terminal device and so on, since the cold cathode tube, requiring an inverter and so on, is too large in the power consumption and results in largely consuming a battery, instead of the cold cathode tube a light source that uses an EL (electro-luminescence) element has been widely adopted.
However, when an EL element is used as a light source of this kind of a display device, in order to obtain brightness of substantially several Cd/m2, power consumption such large as 4 to 5 mW/cm2 is necessary, accordingly there is a problem in that despite of larger power consumption, the brightness is low.
Furthermore, in the EL element, in order to allow the element itself to emit, an inverter is necessary in an amplitude circuit, and a problem results in that the inverter becomes a noise source of a liquid crystal display device. So far, as a countermeasure to the inverter noise, it has been necessary to provide the liquid crystal display device with a noise countermeasure circuit and noise shield means, resulting in higher cost. Furthermore, the inverter, in view of a circuit, is disadvantageous in raising the cost.
Furthermore, since the EL element has disadvantages in that the emission life is relatively short, for a longer use, component replacement and maintenance are necessary. For instance, in the case of an ordinary EL element, in terms of a half decay time when brightness becomes one half the initial brightness, the emission life is substantially 5000 h at the longest and substantially 2500 h at the shortest. Accordingly, a light source that has increased lifetime is in demand.
The present invention provides a backlight that is lower in power consumption, exhibits higher brightness, has a longer lifetime and generates less noise, and a liquid crystal display device provided therewith.